1. Field of the Invention
Embodiments of the present invention generally relate to a pressure vessel skirt for accommodating thermal cycling.
2. Description of the Related Art
Delayed petroleum coking is a process in which a petroleum fraction is heated to a temperature at which it thermally decomposes to provide a solid coke product and a hydrocarbon distillate product. In general, a liquid petroleum feed stock is first distilled until the lighter ends have been recovered and a heavy residuum remains. This heavy residuum of heated pitch and cat-cracked heavy or cycle oil is charged to the bottom of a structure called a coke drum.
Coke drums are vertically-disposed pressure vessels that are commonly twelve to thirty-two feet in diameter with a thirty to over eighty-feet tall cylindrical section. A coke drum typically has a conical bottom section to provide uniformity of support stresses in the structure, and is supported by a skirt that is welded to or near a transition knuckle between the cylindrical section and the conical bottom section.
In the coke drum, the heavy residuum is further heated to about one thousand degrees Fahrenheit and undergoes extensive and controlled cracking and coking under high-pressure conditions. A cracked lighter product rises to the top of the coke drum in a process called steam stripping and is drawn off.
A heavier product remains and cracks to coke, a solid, coal-like substance. The coke is usually purged with steam to remove any remaining volatile components. After the cracking and coking process is complete, quench water is introduced and high-pressure water jets are used to cut away and remove the coke. The water reduces the temperature in the drum to around two hundred degrees Fahrenheit or less before a new cycle begins. In order to increase production speed, the quenching operation is often done as quickly as possible. The cycle time for a vessel is typically 48 hours or less.
The heating and quenching cyclic operations of vessels such as coke drums cause deterioration of the structure over time. Vessels subjected to such extreme thermal cycling may experience a failure in the area where the support skirt is welded to the vessel shell. Cracking and structural failure of the support skirt, the vessel wall, and/or the attachment weld may occur.
A structure that is more resistant to cracking and other fatigue-related failures would be advantageous.